Well, for old pokemon, nothing will ever beat Gallade for me. Psychic/Fighting, move tutor person lets him learn all element variants of Slash, and he has SWORD ELBOWS!
Fairly early into playthrough of Black, however, I discover the awesome that is Sawk. Its double kick has yet to fail me and it has been a permanent fixture of my party (except during the bird gym, stupid birds)
I thought that would be all but then I come face to face with Cubchoo. I literally just stared at it for a full minute before declaring "I will catch it and love it and name it George." Just look at it!
So there's my favorites.